1. Field of the Invention
This invention relates to a method and apparatus for treating exhaust gas for removal of fine particles, more particularly to such a method and apparatus capable of highly effective removal of non-volatile dry soot as well as of soluble organic fractions (SOF) apt to escape upon volatilization.
2. Prior Art Statement
The fine particles of noxious substances contained in the exhaust gas from diesel engines, gas turbines, stirling engines and the like are polluting the environment.
Conventional techniques for reducing the fine-particle content of exhaust gas center on methods for improving combustion and methods for treatment of the exhaust gas after it has been produced. This invention falls in the latter category.
One such method for treating exhaust gas after it has been produced involves the use of a filter trap. A filter trap made of ceramic or the like is installed downstream of the exhaust manifold and the fine particles are trapped as the exhaust gas passes therethrough. When the quantity of the trapped particles reaches such a level as to cause the engine exhaust pressure to fall below a predetermined level, the filter is heated so as to burn the trapped fine particles and regenerate the filter. This process is repeated intermittently.
In order to burn off the trapped fine particles at the time of filter regeneration, it is necessary to establish the required combustion conditions within a short period of time. Specifically, an appropriate oxygen concentration and temperature for commencement of combustion must be quickly established. Conventionally, therefore, the filter has been installed immediately downstream of the exhaust manifold and the exterior of the filter has been heat insulated so as to take advantage of the heat of the exhaust gas and thus make it possible to achieve the required temperature increase rapidly and to minimize energy consumption.
In the conventional system it is therefore necessary to carry out fine particle trapping at a relatively high temperature, with the result that the temperature frequently rises above the volatilization temperature of the soluble organic: fractions (SOF) contained in the fine particles. As a result, most of the SOF has been exhausted in the form of gas, making it impossible to trap it in the filter.
Moreover, exhaust gas continues to pass through the filter even during the regeneration process. Thus even in the case where fine particles including SOF components are trapped, those SOF components which volatilize before the temperature for burning the fine particles is reached are exhausted together with the gas passing through the filter.
This inability to remove SOF components has seriously detracted from the effectiveness of the conventional exhaust gas treatment methods and apparatus as regards their ability to reduce the emission of noxious fine particles.
While it is possible to employ a metallic-system catalyst in order to carry out the filter generation at a relatively low temperature, i.e. for lowering the fine particle combustion temperature, this has the adverse effect of increasing the emission of sulfates.